#include <stdio.h>
#include <limits.h>  //提供CHAR_BIT的定义，CHAR_BIT表示每字节的位数

char *itobs(int, char *);

void show_bstr(const char *);

int invert_end(int num,int bits);

int main() {
    char bin_str[CHAR_BIT * sizeof(int) + 1];
    int number;

    puts("Enter integers and see them in binary.");
    puts("Non-numeric input terminates program.");
    while (scanf("%d", &number) == 1) {
        itobs(number, bin_str);
        printf("%d is\n", number);
        show_bstr(bin_str);
        putchar('\n');
        number = invert_end(number,4);
        printf("Inverting the last 4 bits gives\n");
        show_bstr(itobs(number,bin_str));
        putchar('\n');
    }
    puts("Bye!");

    return 0;
}

char *itobs(int n, char *ps) {
    int i;
    const static int size = CHAR_BIT * sizeof(int);

    for (i = size - 1; i >= 0; i--, n >>= 1)
        ps[i] = (01 & n) + '0';
    ps[size] = '0';

    return ps;
}

/*4位一组显示二进制字符串*/
void show_bstr(const char *str) {
    int i = 0;

    while (str[i])  /* 不是一个空字符 */
    {
        putchar(str[i]);
        if(++i % 4 == 0 && str[i])
            putchar( ' ');
    }
}

int invert_end(int num, int bits){
    int mask = 0;
    int bitval = 1;

    while (bits--> 0){
        mask |= bitval;
        bitval <<= 1;

    }
    return num ^ mask;
}